For the purpose of regional and global environment protection, ceramic honeycomb structures are used in exhaust-gas-cleaning catalyst converters to reduce harmful materials contained in exhaust gases from automobile engines, etc. To capture fine graphite particles, etc. contained in exhaust gas from diesel engines, exhaust-gas-cleaning filters comprising porous ceramic honeycomb structures with both cell ends alternately plugged are recently used. As shown in FIGS. 3(a) and 3(b), a ceramic honeycomb structure 31 comprises a peripheral wall 36, and cell walls 34 perpendicularly crossing each other inside the peripheral wall 36 for defining large numbers of cells 35. As shown in FIGS. 3(c) and 3(d), a ceramic honeycomb filter 30 for cleaning an exhaust gas from automobiles by capturing particulate matter comprises a ceramic honeycomb structure 31 whose both end surfaces 32a, 32b are alternately sealed by plugs 33a, 33b in a checkerboard pattern.
The ceramic honeycomb structure 31 is produced by the steps of (1) preparing a moldable material by weighing, mixing and blending materials such as a ceramic powder, a binder, etc., (2) extruding the moldable material through a honeycomb die, (3) cutting the extruded moldable material to form a honeycomb-structure, molded body (simply called “molded body”), (4) drying and sintering the molded body, (5) machining end surfaces 32a, 32b of the dried or sintered molded body with a tool such as a diamond cutter, a diamond saw, etc. to form a ceramic honeycomb structure 31 having a predetermined length L, (6) sealing cells 35a, 35b at both end surfaces 32a, 32b of the ceramic honeycomb structure 31 shown in FIG. 3(a) with plugs 33a, 33b, and (7) sintering the plugs to form a ceramic honeycomb filter 30 shown in FIG. 3(c). The sealing step (6) may be conducted after drying the molded body or after sintering the dried body.
In the step (5) of forming the ceramic honeycomb structure 31, as shown in FIG. 4, when the end surfaces 32a, 32b are machined by a tool, dust and cut pieces 40 are attached to the end surfaces 32a, 32b and cell walls 34 near the end surfaces 32a, 32b. Dust and cut pieces attached to the end surfaces 32a, 32b and the cells 35a, 35b have been removed by a vacuum cleaner, etc. manually and carefully. Such manual method, however, fails to remove dust and cut pieces attached to the cells deep from the end surfaces. Further, because the ceramic honeycomb structure has relatively low impact strength, its cell walls on the end surfaces are damaged and broken during removing dust and cut pieces. Accordingly, carefulness is needed to avoid damaging the cell walls, and a manual method is extremely inefficient particularly for large ceramic honeycomb structures 31 of 20 cm or more in diameter.
When dust remains in the cells of the dried ceramic honeycomb structure, the dust is adhered to the cell walls in a subsequent sintering step, providing the honeycomb structure with large pressure loss. When dust remains in the cells of the sintered ceramic honeycomb structure, a catalyst cannot be effectively carried in a subsequent catalyst-carrying step. Also, when used as a ceramic honeycomb filter, the dust is adhered to the cell walls in a plug-sintering step, providing the ceramic honeycomb filter with large pressure loss.
The damaging and breakage of the cell walls 34 on the end surfaces 32a, 32b provide the ceramic honeycomb structure with low strength and reliability. When used as a ceramic honeycomb filter, a plugging material charged into the cells flows into adjacent cells through the broken cell walls, resulting in large pressure loss.
As a method for mechanically removing dust and cut pieces from end surface portions of the ceramic honeycomb structure, JP 8-117713 A discloses an apparatus 51a for cleaning end surface portions of a honeycomb structure as shown in FIG. 5(a). The apparatus 51a comprises a spray nozzle 53a with a brush 52a, which is opposing an end surface 32a of the ceramic honeycomb structure 31, and vertically movable in parallel with the end surface 32a, a narrow dust-collecting hopper 54a disposed at an end surface 32b on the opposite side of the end surface 32a, a carrier 55a for rotating 180° a table on which the ceramic honeycomb structure is placed, and a mechanism for moving the spray nozzle 53a with the brush 52a vertically, such that dust and cut pieces are detached from the end surface 32a by the brush 52a and blown off by air ejected from the spray nozzle 53a. In FIG. 5(a) and FIG. 5(b) mentioned later, the same reference numerals are assigned to substantially the same members as in FIG. 3. JP 8-117713 A describes that dust and cut pieces can be easily removed from the end surface 32a (and 32b) of the ceramic honeycomb structure by the cleaning apparatus 51a, and collected without scattering by the dust-collecting hopper 54a directly connected to a dust-collecting machine 54, thereby preventing the deterioration of working environment.
JP 2000-43024 A discloses, as shown in FIG. 5(b), a machining apparatus 51b for removing fine powder and burr generated by cutting from a end surface portion, by blowing air onto cells on an end surface 32b of a honeycomb structure 31, while brushing an end surface 32a with a brush 52b. JP 2000-43024 A describes that the machining apparatus 51b can completely remove burr, etc. from the end surface 32a of the honeycomb structure 31 without damaging the end surface 32a. 
However, the methods described in JP 8-117713 A and JP 2000-43024 A conduct brushing while blowing air onto the end surfaces 32a, 32b of the honeycomb structure, likely damaging the cell walls 34. Particularly, ceramic honeycomb structures as shown in FIG. 3(a) recently used for exhaust-gas-cleaning catalyst converters comprise cell walls as thin as 0.15 mm or less to have small heat capacity to accelerate the activation of a catalyst even at the time of cold start, and ceramic honeycomb structures 31 used for the ceramic honeycomb filters 30 as shown in FIG. 3(c) comprise cell walls 34 having as high porosity as 50-80%. Because the ceramic honeycomb structures having such low heat capacity or high porosity comprise extremely-low-strength cell walls 34, the brushing of the end surfaces 32a, 32b as in the above methods described in JP 8-117713 A and JP 2000-43024 A are likely to damage cell walls 34 on the end surfaces 32a, 32b. 
Attempts to remove dust and cut pieces only with air blow without using a brush to prevent cell walls near the end surfaces from being damaged by brushing fail to achieve efficient removal of the dust and cut pieces, resulting in poor quality with dust and cut pieces remained, and take a long period of time for removal, resulting in high cost.